Non-linearity of the solid-electrolyte-interphase overpotential

Unveiling the Mystery of LiF within Solid Electrolyte Interphase in Lithium Batteries

Over the past decades, significant advances have been made in lithium-ion batteries. However, further requirement on the electrochemical performance is still a powerful motivator to improve battery technology. The solid electrolyte interphase (SEI) is considered as a key component on negative electrode, having been proven to be crucial for the performance, even in safety of batteries. Although numerous studies have focused on SEI in recent years, its specific properties, including structure and composition, remain largely unclear. Particularly, LiF, a common and important component in SEI, has sparked debates among researchers, resulting in divergent viewpoints. In this review, the recent research findings on SEI and delve into the characteristics of the LiF component is aim to consolidated. The cause of SEI formation and the evolution of SEI models is summarized. The distinctive properties of SEI generated on various negative electrodes is further discussed, the ongoing scholarly controversy surrounding the function of LiF within SEI, and the specific physicochemical properties about LiF and its synergistic effect in heterogeneous components. The objective is to facilitate better understanding of SEI and the role of the LiF component, ultimately contributing to the development of Li batteries with enhanced electrochemical performance and safety for battery communities.

Study on the electrical-thermal properties of lithium-ion battery materials in the NCM622/graphite system

The phenomenon of fire or even explosion caused by thermal runaway of lithium-ion power batteries poses a serious threat to the safety of electric vehicles. An in-depth study of the core-material thermal runaway reaction mechanism and reaction chain is a prerequisite for proposing a mechanism to prevent battery thermal runaway and enhance battery safety. In this study, based on a 24 Ah commercial Li(Ni0.6Co0.2Mn0.2)O2/graphite soft pack battery, the heat production characteristics of different state of charge (SOC) cathode and anode materials, the separator, the electrolyte, and their combinations of the battery were investigated using differential scanning calorimetry. The results show that the reaction between the negative electrode and the electrolyte is the main mode of heat accumulation in the early stage of thermal runaway, and when the heat accumulation causes the temperature to reach a certain critical value, the violent reaction between the positive electrode and the electrolyte is triggered. The extent and timing of the heat production behaviour of the battery host material is closely related to the SOC, and with limited electrolyte content, there is a competitive relationship between the positive and negative electrodes and the electrolyte reaction, leading to different SOC batteries exhibiting different heat production characteristics. In addition, the above findings are correlated with the battery failure mechanisms through heating experiments of the battery monomer. The study of the electro-thermal properties of the main materials in this paper provides a strategy for achieving early warning and suppression of thermal runaway in batteries.

Non-Linear Kinetics of The Lithium Metal Anode on Li6 PS5 Cl at High Current Density: Dendrite Growth and the Role of Lithium Microstructure on Creep

Interfacial instability, viz., pore formation in the lithium metal anode (LMA) during discharge leading to high impedance, current focusing induced solid-electrolyte (SE) fracture during charging, and formation/behaviour of the solid-electrolyte interphase (SEI), at the anode, is one of the major hurdles in the development of solid-state batteries (SSBs). Also, understanding cell polarization behaviour at high current density is critical to achieving the goal of fast-charging battery and electric vehicle. Herein, via in situ electrochemical scanning electron microscopy (SEM) measurements, performed with freshly deposited lithium microelectrodes on transgranularly fractured fresh Li6PS5Cl (LPSCl), the LiǀLPSCl interface kinetics are investigated beyond the linear regime. Even at relatively small overvoltages of a few mV, the LiǀLPSCl interface shows non-linear kinetics. The interface kinetics possibly involve multiple rate-limiting processes, i.e., ion transport across the SEI and SE|SEI interfaces, as well as charge transfer across the LiǀSEI interface. The total polarization resistance RP of the microelectrode interface is determined to be ≈ 0.8 Ω cm2 . It is further shown that the nanocrystalline lithium microstructure can lead to a stable LiǀSE interface via Coble creep along with uniform stripping. Also, spatially resolved lithium deposition, i.e., at grain surface flaws, grain boundaries, and flaw-free surfaces, indicates exceptionally high mechanical endurance of flaw-free surfaces toward cathodic load (>150 mA cm-2 ). This highlights the prominent role of surface defects in dendrite growth.

Active material and interphase structures governing performance in sodium and potassium ion batteries

Development of energy storage systems is a topic of broad societal and economic relevance, and lithium ion batteries (LIBs) are currently the most advanced electrochemical energy storage systems. However, concerns on the scarcity of lithium sources and consequently the expected price increase have driven the development of alternative energy storage systems beyond LIBs. In the search for sustainable and cost-effective technologies, sodium ion batteries (SIBs) and potassium ion batteries (PIBs) have attracted considerable attention. Here, a comprehensive review of ongoing studies on electrode materials for SIBs and PIBs is provided in comparison to those for LIBs, which include layered oxides, polyanion compounds and Prussian blue analogues for positive electrode materials, and carbon-based and alloy materials for negative electrode materials. The importance of the crystal structure for electrode materials is discussed with an emphasis placed on intrinsic and dynamic structural properties and electrochemistry associated with alkali metal ions. The key challenges for electrode materials as well as the interface/interphase between the electrolyte and electrode materials, and the corresponding strategies are also examined. The discussion and insights presented in this review can serve as a guide regarding where future investigations of SIBs and PIBs will be directed.

Electrolyte formulation strategies for potassium-based batteries

Potassium (K)-based batteries are viewed as the most promising alternatives to lithium-based batteries, owing to their abundant potassium resource, lower redox potentials (-2.97 V vs. SHE), and low cost. Recently, significant achievements on electrode materials have boosted the development of potassium-based batteries. However, the poor interfacial compatibility between electrode and electrolyte hinders their practical. Hence, rational design of electrolyte/electrode interface by electrolytes is the key to develop K-based batteries. In this review, the principles for formulating organic electrolytes are comprehensively summarized. Then, recent progress of various liquid organic and solid-state K+ electrolytes for potassium-ion batteries and beyond are discussed. Finally, we offer the current challenges that need to be addressed for advanced K-based batteries.

Strategies for Harnessing High Rate and Cycle Performance from Graphite Electrodes in Potassium-Ion Batteries

Potassium-ion batteries (PIBs) have been lauded as the next-generation energy storage systems on account of their high voltage capabilities and low costs and the high abundance of potassium resources. However, the practical utility of PIBs has been heavily encumbered by severe K metal dendrite formation, safety issues, and insufficient electrochemical performance during operations─indeed critical issues that underpin the need for functional electrolytes with high thermal stability, robust solid-electrolyte interphase (SEI)-forming capabilities, and high electrochemical performance. In a bid to establish a knowledge framework for harnessing high rate capabilities and long cycle life from graphite negative electrodes, this study presents the physical properties and electrochemical behavior of a high K⁺ concentration inorganic ionic liquid (IL) electrolyte, K[FSA]-Cs[FSA] (FSA^- = bis(fluorosulfonyl)amide) (54:46 in mol), at an intermediate temperature of 70 °C. This IL electrolyte demonstrates an ionic conductivity of 2.54 mS cm^-1 and a wide electrochemical window of 5.82 V. Charge-discharge tests performed on a graphite negative electrode manifest a high discharge capacity of 278 mAh g^-1 (0.5 C) at 70 °C, a high rate capability (106 mAh g^-1 at 100 C), and a long cyclability (98.7% after 450 cycles). Stable interfacial properties observed by electrochemical impedance spectroscopy during cycling are attributed to the formation of sulfide-rich all-inorganic SEI, which was examined through X-ray photoelectron spectroscopy. The performance of the IL is collated with that of an N-methyl-N-propylpyrrolidinium-based organic IL to provide insight into the synergism between the highly concentrated K⁺ electrolyte at intermediate temperatures and the all-inorganic SEI during electrochemical operations of the graphite negative electrode.

Effect of Crystal Orientation of Cu Current Collectors on Cycling Stability of Li Metal Anodes

Li metal anodes are plagued by low coulombic efficiency due to their interfacial instability. Many approaches were proposed to cope with this problem; however, little attention has been given to the current collector of Li anodes. In this study, we investigate the crystal orientation dependence of the cycling stability of Li anodes on single-crystal Cu(111), (101), and (001) and polycrystalline Cu current collectors. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) show that (111) and (001) achieved high current efficiency and low interfacial resistance, while (101) and polycrystalline Cu exhibited low cyclabilities. X-ray photoelectron spectroscopy (XPS) and auger electron spectroscopy (AES) analysis revealed that the thickness of the solid electrolyte interphase (SEI) varies with the Cu crystal orientation, and the SEI is the thinnest on the single-crystal Cu(111). This tendency can be explained by the orientation dependence of the surface energy of Cu, which corresponds to the chemical activity of the surfaces. Our result advocates the importance of considering Cu orientation for interfacial engineering of Li metal anodes.

Tuning Solid Electrolyte Interphase Layer Properties through the Integration of Conversion Reaction

The solid electrolyte interphase (SEI) layer plays an important role in altering the ion transport and modifying the structural evolution of the Li metal anode during repeated cycling. While the fundamental understanding of the SEI properties has been continuously advanced in recent years, effectively tuning the SEI components, especially the inorganic constituents, is still challenging. In this work, tungsten trioxide, WO₃, is found to promote the formation of inorganic salts, for example, LiF/Li₂CO₃ in SEI layers, thereby enhancing the SEI properties such as mechanical and chemical stabilities. Additionally, WO₃ is simultaneously reduced to electronic W nanoparticles during the electrochemical process, mitigating the formation of "dead" Li, which otherwise is completely wrapped by the accumulated insulating SEI layers. The possibility of WO₃ in catalyzing electrolyte decomposition, through favored reaction pathway, to produce robust SEI layers is discussed. This work provides new insights into the control of the SEI properties on Li metal surfaces.